As seen in FIG. 3, an angle detecting device 9 having a resolver has been conventionally known as one of measured value detecting devices for detecting a measured value based upon an output signal which is independent of a direct-current component. The angle detecting device 9 includes a measuring unit 10 and a detecting unit 20 which are interconnected via an excitation line 51, output transmitting lines 52, 53, and a reference potential line 54. The measuring unit 10 houses a monophasic excitation and biphasic output type (1 phase/2 phase) of resolver possessing a pair of two secondary winding coils 18, 19, and an excitation coil 17. The excitation coil 17 is fixed to a rotor (not shown) rotatable relatively to the secondary coils 18 and 19.
One end of the excitation coil 17 is connected to a ground (hereinafter, referred to as a GND) housed in the detecting unit 20 via the reference potential line 54 and the other end thereof is connected to an excitation signal generating circuit 21 housed in the detecting unit 20 via the excitation line 51. The excitation signal generating circuit 21 is connected to a central processing unit (hereinafter, referred to as a CPU) 22 housed in the detecting unit 20 and receives a command signal from the CPU 22. One ends of the respective secondary coils 18 and 19 are connected to the GND via the reference potential line 54, so that an output reference potential for each coil 18 and 19 is set at zero volts. The other ends of the respective secondary coils 18 and 19 are connected to the output transmitting lines 52 and 53 so as to individually transmit output signals therefrom to the detecting unit 20.
An excitation signal EX is transmitted from the excitation signal generating circuit 21 to the excitation coil 17 based upon the command from the CPU 22. Magnetic field is created in response to electric excitation of the excitation coil 17 applied with the excitation signal EX. Thereby, output signals SS and SC are electrically induced in the secondary coils 18 and 19, respectively. Both of the output signals SS and SC are sine waves, each of which central value is zero volts and amplitude coefficient is SINθ and COSθ corresponding to a rotational angle θ of the rotor. The output signal SS is denoted with “D·SINθ·SINωt” and the output signal SC is denoted with “D·COSθ·SINωt”. The constant numerical “D” is determined based upon a temperature, the amplitude of the excitation signal EX, and the like.
The output signal SS is transmitted to the detecting unit 20 via the output transmitting line 52 so that the central value of the output signal SS with the sine wave varies by resistances R91, R92, and a detecting reference potential V1. The output signal SS is then amplified by a non-inverting amplifying circuit including resistances R93, R94, and an operational amplifier 93. For example, in the case that the detecting unit 20 is provided with the resistance R91 with 5 kΩ, the resistance R92 with 20 kΩ, and the detecting reference potential V1 with 2.5v, the central value at zero volts is changed up to 0.5 volts. The output signal SS is then amplified by the non-inverting amplifying circuit and the central value thereof is changed up to 2.5 volts in the case that the non-inverting amplifying circuit has a gain set to be 5.
The output signal SC is transmitted to the detecting unit 20 via the output transmitting line 53 so that the central value of the output signal SC with the sine wave is changed by resistances R95, R96, and the detecting reference potential V1. The output signal is then amplified by a non-inverting amplifying circuit including resistances R97, R98, and an operational amplifier 94. For example, in the case that the detecting unit 20 is provided with the resistance R95 with 5 kΩ, the resistance R96 with 20 kΩ, and the detecting reference potential V1 with 2.5 volts, the central value of zero volts is changed up to 0.5 volts. The output signal SC is then amplified by the non-inverting amplifying circuit and the central value thereof is changed up to 2.5 volts in the case that the non-inverting amplifying circuit has a gain set to be 5.
The amplified output signals SS and SC are received and converted to digital values by an analog-digital converter 29, which is a known receiving unit. The CPU 22 calculates the rotational angle θ of the rotor based upon the digital values.
However, according to the above-described known angle detecting device 9, when the rotational angle θ of the rotor is zero degree, the SINθ is zero so that an amplitude D·SINθ of the output signal SS will result in zero as well. That is, the output signal SS is zero volts. In this case, it may not have been possible to detect a short-circuit between the output transmitting line 52 for transmitting the output signal SS and the reference potential line 54.
Further, there can be a case that the value of the output signal SS can be the same as the value of the output signal SC depending on a rotational angle θ of the rotor. In this case, it may not have been also possible to detect a short-circuit between the two output signal transmitting lines. Still further, in the case that tie measuring unit can be provided with a plurality pairs of the excitation coil and the secondary coils, output signals on respective plural output transmitting lines can be the same as one another with a high potentiality, thereby short-circuit between the output transmitting lines may not be able to be properly detected.
The present invention therefore seeks to provide an improved angle detecting device surely capable of detecting a short-circuit between an output transmitting line and a reference potential line and a short-circuit between output transmitting lines independently of a rotational angle θ of a rotor.